The invention relates to a voltage regulator, such as a switching voltage regulator.
A DC-to-DC voltage regulator typically is used to convert a DC input voltage to either a higher or a lower DC output voltage. One type of voltage regulator is a switching regulator that is often chosen due to its small size and efficiency. The switching regulator typically includes one or more switches that are rapidly opened and closed to transfer energy between an inductor (a stand-alone inductor or a transformer, as examples) and an input voltage source in a manner that regulates an output voltage.
As an example, referring to FIG. 1, one type of switching regulator is a synchronous Buck switching regulator 10 that receives an input DC voltage (called V.sub.IN) and converts the V.sub.IN voltage to a lower regulated output voltage (called V.sub.OUT) that appears at an output terminal 11. To accomplish this, the regulator 10 may include a switch 20 (a metal-oxide-semiconductor field-effect-transistor (MOSFET), for example) that is operated (via a voltage called V.sub.SW) in a manner to regulate the V.sub.OUT voltage, as described below.
Referring also FIGS. 2 and 3, in particular, the switch 20 opens and closes to control energization/de-energization cycles 19 (each having a constant duration called T.sub.S) of an inductor 14. In each cycle 19, the regulator 10 asserts, or drives high, the V.sub.SW voltage during an on interval (called T.sub.ON) to close the switch 20 and transfer energy from an input voltage source 9 to the inductor 14. During the T.sub.ON interval, a current (called I.sub.L) of the inductor 14 has a positive slope. During an off interval (called T.sub.OFF) of the cycle 19, the regulator 10 deasserts, or drives low, the V.sub.SW voltage to open the switch 20 and isolate the input voltage source 9 from the inductor 14. At this point, the level of the I.sub.L current is not abruptly halted, but rather, a diode 18 begins conducting to transfer energy from the inductor 14 to a bulk capacitor 16 and a load (not shown) that are coupled to the output terminal 11. During the T.sub.OFF interval, the I.sub.L current has a negative slope, and the regulator 10 may close a switch 21 to shunt the diode 18 to reduce the amount of power that is otherwise dissipated by the diode 18. The bulk capacitor 16 serves as a stored energy source that is depleted by the load, and additional energy is transferred from the inductor 14 to the bulk capacitor 16 during each T.sub.ON interval.
For the Buck switching regulator, the ratio of the T.sub.ON interval to the T.sub.OFF interval, called a duty cycle, generally governs the ratio of the V.sub.OUT to the V.sub.IN voltages. Thus, to increase the V.sub.OUT voltage, the duty cycle may be increased, and to decrease the V.sub.OUT voltage, the duty cycle may be decreased.
As an example, the regulator 10 may include a controller 15 (see FIG. 1) that regulates the V.sub.OUT voltage by using a pulse width modulation (PWM) technique to control the duty cycle. In this manner, the controller 15 may include an error amplifier 23 that amplifies the difference between a reference voltage (called V.sub.REF) and a voltage (called V.sub.P (see FIG. 1)) that is proportional to the V.sub.OUT voltage. Referring also to FIG. 5, the controller 15 may include a comparator 26 that compares the resultant amplified voltage (called V.sub.C) with a sawtooth voltage (called V.sub.SAW) and provides the V.sub.SW signal that indicates the result of the comparison. The V.sub.SAW voltage is provided by a sawtooth oscillator 25 and has a constant frequency (i.e., 1/T.sub.S).
Due to the above-described arrangement, when the V.sub.OUT voltage increases, the V.sub.C voltage decreases and causes the duty cycle to decrease to counteract the increase in V.sub.OUT. Conversely, when the V.sub.OUT voltage decreases, the V.sub.C voltage increases and causes the duty cycle to increase to counteract the decrease in V.sub.OUT.
The switching frequency (i.e., 1/T.sub.S) typically controls the magnitude of an AC ripple component (called V.sub.RIPPLE (see FIG. 4)) of the V.sub.OUT voltage, as a higher switching frequency typically means a lower magnitude of the V.sub.RIPPLE voltage. Unfortunately, a higher switching frequency may present difficulties. For example, a higher switching frequency may cause an increase in magnetic core losses (of the inductor 14, for example). As another example, the power dissipated by the switch 20 increases with a higher switching frequency. One way to decrease the power dissipation is to decrease the resistance of the switch 20 when the switch 20 is closed. For example, if the switch 20 is a MOSFET, the on resistance (called Rds(on)) of the MOSFET may be decreased by forming the switch 20 out of multiple MOSFETs that are connected in parallel and/or increasing the size of the MOSFET(s). However, these techniques typically increase the gate capacitance(s) of the MOSFET(s), an effect that may limit the performance of the switch 20.
Thus, there is a continuing need for a switching regulator that achieves the benefits gained from a high switching frequency without incurring the disadvantages.